Page 78 - Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

Rod bundle and pool-type experiments in water serving liquid metal reactors 53

where

3 ν
ð
H gβ T h T c Þ H
Gr ¼ Grashof ; Pr ¼ Prandtl; A R ¼ Geometrysimilarity:
V 2 α L
ch
In analogy with Reynolds in forced convection flows, Grashof number defines the
regime (laminar, transitory, or turbulent) of the natural convection flow under anal-
ysis. The Prandtl number is the ratio between momentum and thermal diffusivity
and represents the main discrepancy between the reactor and models, due to the very
different fluid properties of LBE (Pr ¼ 0.02 at 350°C) and water (Pr ¼ 7at20°C).

3.1.2.3 Fuel bundle investigation
Rod bundle facilities, at room temperature and 1 bar, can be designed to study the flow
field and to perform flow-induced vibration experiments. The results would provide a
useful benchmark for CFD validation. Because the heat transfer is not taken into
account, the scaling laws are much simpler. The geometry of the bundle determines
the length scale of the characteristic features of the flow (i.e., coherent structures,
more in Section 3.1.3) that are often object of the study.
The P/D ratio (Pin pitch-to-Pin diameter ratio) is the parameter, which determines
the lattice of the rod bundle geometry. The equivalent hydraulic diameter of the whole
bundle, as well as the ones of the individual bundle subchannels, depends on the P/D
ratios. Once the geometry is fixed, the focus moves to the Reynolds number. It rep-
resents the second main constraint of the rod bundle design. The Reynolds number is a
parameter of interest for experiments, as it affects the physics of the studied phenom-
ena, and for CFD simulations that aim to use the benchmark as reference for valida-
tion. Both the P/D ratio and the Reynolds determine the required flow rate, which is
important for the choice of the pump used to feed the loop.

3.1.2.4 Experimental techniques
In this section the main experimental (optical) techniques are presented, including
some issues arising from their practical application.

3.1.2.4.1 Laser Doppler anemometry
LDA is a nonintrusive single-point optical measurement technique suitable for bundle
flow experiments. The measurement system consists of a laser beam, which is split
with a Bragg cell; the beam pair passes through a lens so that the two beams cross
each other at the focal point. Here the lasers form an ellipsoidal region of interfering
fringes, which represents the measurement volume (also referred to as measurement
probe) where the passage of tracer particles is sensed. Fig. 3.1.1 illustrates the system.
The distance between the fringes can be calculated as

λ
d f ¼ ,
2sinðθ=2Þ